The bones and connective tissue of an adult human spinal column consists of more than 20 discrete bones coupled sequentially to one another by a tri-joint complex. The complex consists of an anterior disc and two posterior facet joints. The anterior discs of adjacent bones are cushioned by cartilage spacers referred to as intervertebral discs. The over 20 bones of the spinal column are anatomically categorized as one of four classification: cervical, thoracic, lumbar, or sacral. The cervical portion of the spine which comprises the top of the spine up to the base of the skull, includes the first 7 vertebrae. The intermediate 12 bones are thoracic vertebrae, and connect to the lower spine comprising the 5 lumbar vertebrae. The base of the spine is a sacral bones (including the coccyx).
The spinal column of bones is highly complex in that it includes the over 20 bones coupled to one another, housing and protecting critical elements of the nervous system having innumerable peripheral nerves and circulatory bodies in close proximity. Despite its complexity, the spine is a highly flexible structure, capable of a high degree of curvature and twist in nearly every direction.
Genetic or developmental irregularities, trauma, chronic stress, tumors and disease, however, can result in spinal pathologies which either limit this range of motion, or which threatens the critical elements of the nervous system housed within the spinal column. A variety of systems have been disclosed in the art which achieve this immobilization by implanting artificial assemblies in or on the spinal column. These assemblies may be classified as anterior, posterior or lateral implants. Lateral and anterior assemblies are coupled to the anterior portion of the spine which is in the sequence of vertebral bodies. Posterior implants generally comprise pairs of rods, which are aligned along the axis which the bones are to be disposed, and which are then attached to the spinal column by either hooks which couple to the lamina or attach to the transverse processes, or by screws which are inserted through the pedicles. In order to provide enhanced torsional rigidity, these implants generally include cross-connecting devices which couple the rods together transverse to the axis of the implants. These cross-connecting devices may couple directly to the rods themselves, or may be attached to the pedicle screws.
There is limited space about the spinal column for the attachment of the screws or hooks which hold in place the implant rods. Factors such as pedicle screw depth, placement and angle of insertion are taken into account when attempting to place implant rods in an optimal manner; however, there is often a trade-off between space requirements, needs inherent to the particular corrective treatment or condition involved, and structural strength and stability. For example, using multi axial pedicle screws (i.e., which allow rotation or deflection of the head thereof in multiple directions) to hold in place implant rods, facilitates surgical manipulation and placement of the rods, but lower strength, as such capacity for movement may remain post-surgery. Such movement may be desired when addressing the needs of certain patients; however, where stronger fixation is desirable concerns as to screw, hook and rod placement are nonetheless applicable. Known mono axial screws may be employed to achieve greater strength, but they do not provide the level or flexibility for manipulation to optimally place the rods as do their multi axial counterparts.
Similarly, when employing cross-linking hook members known in the art, concerns arise due to the tight quarters for placement of same, particularly if the given surgical scenario requires or would benefit from use of cross-linking members that traverse a non-linear path between adjacent rods whilst still achieving a high level of added strength and stability, and flexibility in terms ease or even possibility of installation.
It is desirable to provide pedicle screws and surgical hook apparatuses that obviate or eliminate the need to make the trade-offs described above.